The aim of this research is to elucidate mechanisms of immunosuppression associated with viral infection. To this end virus-induced changes in the life history and function of lymphocytes would be studied in vivo and in vitro using cell separation techniques, labeling methods and quantitative assays that measure the immunological activity of defined lymphocyte subsets. Defects in lymphocyte function revealed after in vitro exposure of the cells to viruses would be related, where possible, to the performance of these lymphocytes in virus-infected rodents, particularly rats and mice. Here methods developed in earlier studies of infection immunity, transplantation immunity and the antibody response would be used to evaluate the immune competence of normal mice, virus-infected subjects and convalescent animals. Prominence would be given to studies which might be expected to disclose the specificity of viruses for individual lymphocyte classes as defined by morphologic, cytokinetic and surface membrane markers; the ability of viruses to influence lymphocyte traffic in the body, their capacity to release "mediators" and their ability to operate alone or in concert with other lymphocytes or macrophages in the induction or expression of immunity. An effort will be made to relate these effects to virus adsorbtion, virus replication and virus-induced changes in the lymphocyte surface.